Enforces that the validation layers should be working pretty much read-
only with the exeption of updating caches. Requires a few tricks:

 - updates EP code generation to add 'const' to pointer parameters
 - enables a kludge const_cast to enable the robust query extension
 - makes some members of Framebuffer mutable to work around syncState
 - makes 'is' queries and other methods in Context/State const

Will allow us to more safely expose the no_error extension.

Bug: angleproject:1280
Change-Id: Id9756757854c9e68fc096ecec8d93759fbe6b3a4
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2060689
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Jonah Ryan-Davis <jonahr@google.com>
Reviewed-by: Geoff Lang <geofflang@chromium.org>